Re: [PATCH v3] mm/vmalloc: lock contention optimization under multi-threading

Baoquan He
2024-02-22T15:36:51Z

On 02/22/24 at 01:52pm, Uladzislau Rezki wrote:
> Hello, Rulinhuang!
>
> > Hi Uladzislau and Andrew, we have rebased it(Patch v4) on branch
> > mm-unstable and remeasured it. Could you kindly help confirm if
> > this is the right base to work on?
> > Compared to the previous result at kernel v6.7 with a 5% performance
> > gain on intel icelake(160 vcpu), we only had a 0.6% with this commit
> > base. But we think our modification still has some significance. On
> > the one hand, this does reduce a critical section. On the other hand,
> > we have a 4% performance gain on intel sapphire rapids(224 vcpu),
> > which suggests more performance improvement would likely be achieved
> > when the core count of processors increases to hundreds or
> > even thousands.
> > Thank you again for your comments.
> >
> According to the patch that was a correct rebase. Right a small delta
> on your 160 CPUs is because of removing a contention. As for bigger
> systems it is bigger impact, like you point here on your 224 vcpu
> results where you see %4 perf improvement.
>
> So we should fix it. But the way how it is fixed is not optimal from
> my point of view, because the patch that is in question spreads the
> internals from alloc_vmap_area(), like inserting busy area, across
> many parts now.

I happened to walk into this thread and come up with one draft patch.
Please help check if it's ok.

From 0112e39b3a8454a288e1bcece220c4599bac5326 Mon Sep 17 00:00:00 2001
From: Baoquan He <bhe@redhat.com>
Date: Thu, 22 Feb 2024 23:26:59 +0800
Subject: [PATCH] mm/vmalloc.c: avoid repeatedly requiring lock unnecessarily
Content-type: text/plain

By moving setup_vmalloc_vm() into alloc_vmap_area(), we can reduce
requiring lock one time in short time.

Signed-off-by: Baoquan He <bhe@redhat.com>
---
mm/vmalloc.c | 23 ++++++++---------------
1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index aeee71349157..6bda3c06b484 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1848,7 +1848,10 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
unsigned long align,
unsigned long vstart, unsigned long vend,
int node, gfp_t gfp_mask,
- unsigned long va_flags)
+ unsigned long va_flags,
+ struct vm_struct *vm,
+ unsigned long vm_flags,
+ const void *caller)
{
struct vmap_node *vn;
struct vmap_area *va;
@@ -1915,6 +1918,8 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,

spin_lock(&vn->busy.lock);
insert_vmap_area(va, &vn->busy.root, &vn->busy.head);
+ if (!(va_flags & VMAP_RAM) && vm)
+ setup_vmalloc_vm(vm, va, vm_flags, caller);
spin_unlock(&vn->busy.lock);

BUG_ON(!IS_ALIGNED(va->va_start, align));
@@ -2947,7 +2952,7 @@ void __init vm_area_register_early(struct vm_struct *vm, size_t align)
kasan_populate_early_vm_area_shadow(vm->addr, vm->size);
}

-static inline void setup_vmalloc_vm_locked(struct vm_struct *vm,
+static inline void setup_vmalloc_vm(struct vm_struct *vm,
struct vmap_area *va, unsigned long flags, const void *caller)
{
vm->flags = flags;
@@ -2957,16 +2962,6 @@ static inline void setup_vmalloc_vm_locked(struct vm_struct *vm,
va->vm = vm;
}

-static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
- unsigned long flags, const void *caller)
-{
- struct vmap_node *vn = addr_to_node(va->va_start);
-
- spin_lock(&vn->busy.lock);
- setup_vmalloc_vm_locked(vm, va, flags, caller);
- spin_unlock(&vn->busy.lock);
-}
-
static void clear_vm_uninitialized_flag(struct vm_struct *vm)
{
/*
@@ -3009,8 +3004,6 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
return NULL;
}

- setup_vmalloc_vm(area, va, flags, caller);
-
/*
* Mark pages for non-VM_ALLOC mappings as accessible. Do it now as a
* best-effort approach, as they can be mapped outside of vmalloc code.
@@ -4586,7 +4579,7 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,

spin_lock(&vn->busy.lock);
insert_vmap_area(vas[area], &vn->busy.root, &vn->busy.head);
- setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
+ setup_vmalloc_vm(vms[area], vas[area], VM_ALLOC,
pcpu_get_vm_areas);
spin_unlock(&vn->busy.lock);
}
--
2.41.0